1. Field of the Invention
The present invention relates, in general, to optical recording apparatuses using a scanning mirror for generating a two-dimensional beam and, more particularly, to a holographic optical recording apparatus, which can convert a one-dimensional signal beam into a two-dimensional signal beam using a scanning mirror at the time of recording data on a holographic recording medium, etc.
2. Description of the Related Art
Recently, a holographic digital data storage system using a semiconductor laser, a Charge Coupled Device (CCD), a Liquid Crystal Display (LCD), etc. has been actively researched/developed. Since the holographic digital data storage system is advantageous in that it has a large storage capacity and very high data transfer rate, not only is it utilized for fingerprint recognition devices for storing and reproducing fingerprints, display devices, etc., but also the application fields thereof have been gradually extended.
Such a holographic digital data storage system allows object light, transmitted from an object, and reference light to interfere with each other, records interference patterns generated due to the interference in a storage medium, such as an optical refractive crystal or polymer that differently reacts to the amplitude and phase of interference patterns, and stores three-dimensional holographic digital data in pages each composed of binary data.
Further, the holographic digital data storage system reproduces the stored three-dimensional data by intercepting the object light and providing only the reference light to the storage medium. The holographic digital data are generally recorded and reproduced in pages in the form of rectangular image data having the shape identical to that of a display screen. However, since alignment is required for precise reproduction, alignment marks are formed on the edges of a holographic data page. In this case, when the alignment marks on the data page are formed as images on a CCD in a pixel-to-pixel manner, light spreads to a neighboring pixel due to the alignment marks if the alignment is not precisely performed, thus causing a problem in that it is impossible to precisely measure alignment. In order to solve the problem, U.S. Pat. No. 6,064,586 proposes a new alignment method for holographic data storage and retrieval, in which alignment marks are boldly indicated on opposite vertical lines formed on the pixels of a holographic data page by three columns, and alignment is performed using the boldly indicated marks. However, there is a limitation in that it is difficult to precisely measure alignment using the alignment marks on pixels due to the characteristics of the holographic data recording and reproduction apparatus that measures alignment within a range of ±0.5 pixel.
In order to overcome the limitation, as shown in FIG. 1, Korean Pat. Appl. No. 2002-30147 proposes a holographic data recording and reproduction apparatus, which obtains the regression lines of edges in each page using Fourier approximation with respect to pixels inserted to perform alignment in a data page, and controls an actuator for adjusting a reference light angle depending on the pixels in each page, the location of regression lines and the difference in slopes, thus automatically controlling the alignment of the data page.
FIG. 1 is a view showing the construction of a conventional holographic data recording and reproduction apparatus.
Referring to FIG. 1, the conventional holographic data recording and reproduction apparatus includes a light source 100, an optoisolator 102, shutters 104 and 110, reflectors 106 and 112, a spatial light modulator 114, an actuator 108, a storage medium 116, a CCD 118, a microcomputer 120, a servo control unit 122, and an image compensation processing unit 124.
A process of automatically aligning data pages in the conventional holographic data recording and reproduction apparatus having the above construction is described.
First, the conventional holographic data recording and reproduction apparatus radiates only a reference beam having a set recording angle onto the storage medium 116 at the time of data reproduction, reproduces a holographic digital data page and transmits the holographic digital data page to the CCD 118. Then, the microcomputer 120 selects one row or column from the data page transmitted from the CCD 118, and performs automatic alignment for the data page. The microcomputer 120 approximates the alignment mark region of the selected row or column using the continuous function of row or column data values.
That is, the microcomputer 120 approximates the alignment mark region using the continuous function of the row or column data values of the data page so as to process a holographic image at a sub-pixel level. At this time, the Fourier approximation is used as the approximation method, and it is noted in the Fourier approximation that the number of harmonics must be equal to or less than ½ of the number of data values.
Further, when a vertical alignment mark is intended to be measured using row pixels, an angle approaches 90°, so that a term of the partial differentiation of y should be deleted. Next, the microcomputer 120 performs second order differentiation with respect to the approximated function and obtains the first or second edge value of the data page. In order to detect the edge of the data page, the second order differentiation is used, and a point where the value, obtained from the second order differentiation of the approximated function, becomes “0”, that is, an inflection point, is an edge. That is, the first edge value is a value when the approximated function is maximal and becomes “0”, and indicates a left edge. The second edge value is a value when the approximated function is minimal and becomes “0”, and indicates a right edge.
In the meantime, the conventional holographic data recording and reproduction apparatus may use both first and second edge values, or any one of them at the time of obtaining the edge values of the data page.
The microcomputer 120 selects a row or column from the data page, obtains each approximated function from the row following the selected row to the last row, or from the column following the selected column to the last column, performs second order differentiation with respect to each approximated function, and obtains each first or second edge value. Next, the microcomputer 120 obtains the regression lines of the first and second edge values of the rows or columns using a fitting method, such as a least squares method. If an alignment mark to be measured is precisely in the center of the left and right edges, the alignment mark is obtained by calculating a mean of the left and right edges.
If the location or slope of the regression line does not have a distance or slope set based on a predetermined position on the data page, the microcomputer 120 controls the server control unit 122 of the actuator 108, adjusting the angle of a reference beam, to automatically align a holographic data page.
For example, if the measured location of the regression line deviates from a distance of a normal data page added to 0.5 pixels by 7 pixels, the servo control unit 122 causes the actuator 108 to be moved by −7 pixels and the data page to be reproduced. Then, the data page reproduced by the CCD 118 is precisely reproduced within a 0.5 pixel range.
In the meantime, if the location or slope of the regression line does not have a distance or slope set based on a predetermined location on the data page, the microcomputer 120 transmits the data page to the image compensation processing unit 124 and executes digital signal processing, thus compensating for the image on the holographic data page. That is, the image compensation processing unit 124 moves the image on the data page by a difference between the measured location or slope of the regression line and the location or slope set based on the predetermined location on the data page, so that automatic alignment can be performed.
In the meantime, the above-described conventional holographic optical recording apparatus uses a scheme of spatially modulating information recorded on a holographic recording medium into two-dimensional information using a two-dimensional liquid crystal, a micron mirror array, a Grating Light Valve (GLV), etc. However, if the above scheme is used, there are several problems in that a large number of manufacturing processes are required and circuit construction is complicated when a two-dimensional spatial light modulator is modulated.